The present invention relates to new calcium soaps possessing a high basicity reserve, more commonly called superbase calcium soaps. These superbase calcium soaps may be used in many applications, especially an anticorrosion additives in lubricants. The present invention also relates to a method for preparing these superbase calcium soaps, as well as to the lubricating compositions containing such superbase calcium soaps.
The most widely known superbase calcium soaps are salts of alkylarylsulphonic acids. These are compounds which are difficult to prepare and to employ. A traditional process for preparing these compounds consists in reacting an alkylarylsulphonic acid with a metal oxide or hydroxide in a mineral oil. The reaction takes place in the presence of carbon dioxide (or CO.sub.2) and in the presence of promoters which make the CO.sub.2 easier to fix. The promoters are labile hydrogen compounds such as phenols, alcohols and aminoalcohols. When the reaction has ended, a cloudy solution is obtained and is purified by centrifuging or filtration. The precipitate obtained in this manner contains, in particular, acid in the form of salt which is extracted in order to minimize the losses of acid (U.S. Pat. No. 4,225,509). Processes of this kind lead to calcium superbase salts of alkylarylsulphonic acids which have a TBN (or Total Basic Number) greater than or equal to 200, and capable of going up to 350. TBN is intended to define the basicity reserve of a superbase soap. The TBN of a superbase soap is the potassium hydroxide equivalent corresponding to one gram of soap when its basicity is titrated with a strong acid. It is expressed in milligrams of potassium hydroxide per gram of calcium superbase salt. This value is defined in accordance with the ASTM standard D 2896-73. Another major disadvantage of superbase calcium soaps of alkylarylsulphonic acids lies in the fact that they yield cloudy solutions which make them awkward to use.